1. Field of the Invention
This invention provides for quantum dot optoelectronic devices (“QD-OEDs”) having enhanced performance, with a particular focus on quantum dot light-emitting devices (“QD-LEDs”) with enhanced performance. However, the benefits of the current invention also apply to quantum dot photovoltaic devices. Compared to the standard QD-OED, the current invention exhibits nominally equal current densities for given applied voltages, a higher internal fluorescence quantum yield (around 8%), increased total light output at equal voltage (around 28%), a lower turn on voltage (around 25%), and a lower operating voltage (around 20%).
The fabrication method for the current invention is similar to the one for the standard device, with one additional step. In particular, a physical surface treatment is performed on the quantum dots (“QDs”) prior to deposition of additional charge injection layers or the second electrode. This physical surface treatment modifies the surface of the QDs.
The surface modification of the current invention can be achieved through the utilization of either a low-power oxygen (O2) plasma treatment, or UV-Ozone treatment, following spin-coating the QD layer. This modification entails the removal of the outer organic capping layer which shrouds the QDs. Elimination of this electrically insulating layer results in improved carrier injection from the charge injection layer, or the electrode, to the outermost QD monolayer, or multilayer, adjacent to the electrode. This also results in better charge balance within the device. The afore-mentioned modification and the method of implementation are applicable to a wide range of quantum dots, including size-dependent or composition-dependent QDs of varying sizes and compositions, as well as core, core-shell, alloyed core, and alloyed core-shell quantum dots. The method is also applicable to both light emitting devices and photovoltaic devices for achieving performance enhancement. In the case of a QD photovoltaic device, removal of the capping layer will improve charge transfer to the electrode (reverse process) and the overall charge collection efficiency.
2. Description of Related Art
Semiconductor quantum dots have attracted much attention due to their unique physical, chemical, electrical and optical properties. Much of the interest in optical and electrical characteristics stems from size-dependent properties owing to quantum confinement of charge carriers. This often results in the ability to “tune” the optical spectrum and specifically, both the light absorption and emission responses (perceived color) through changing the size of the QDs. Recently, QDs have been explored for their electroluminescent properties with applications in optoelectronics, particularly, as active emitting layers in planar light emitting devices. Quantum dots have also been studied for use in photovoltaics, specifically as the active layer in solar cells.
Conventional planar light emitting devices such as organic light emitting diodes (OLED) incorporate organic materials as the active emitting layer. This active layer is typically sandwiched between two electrodes, along with other charge injection and charge transport layers. Applying an external electric field through the electrodes results in injection and migration of carriers and formation of electron/hole pairs (excitons) within or at the interface of the active emitting layer, which following radiative recombination, results in emission of (typically) visible light. The color of the emitted light is tuned through changing the composition of the active organic layer or additional filter layers to afford red, green, and blue emission suitable for display or lighting applications.
Planar LED devices that utilize quantum dots, e.g. QD-LEDs, have a structure very similar to traditional OLEDs, with the exception of substitution of inorganic QDs as the active emissive layer in place of the organic materials. Advantages of this methodology include potentially higher material stability, better lifetime and efficiency, as well as increased color saturation, spectral tunability, and relative ease of material preparation. QDs of different sizes, exhibiting different spectral responses, have been prepared and deposited via spin-coating, forming a thin film with a thickness ranging from a single monolayer to several tens of nanometers (nm) within a traditional OLED-type structure. Although these devices to date have not achieved the power efficiencies of their all-organic counterpart, they have exhibited the potential and as such, much effort has been focused on enhancing the characteristics of the QD-LED devices.
Nanostructured solar cells exemplify the novel trend in solar technology research and development, the so-called “Third Generation”. This generation of solar cells includes technologies utilizing nanomaterials, polymers, and organic molecules to engineer efficient junctions, provide self-assembly, and form ultra thin films. This provides significant opportunities to achieve low cost, flexibility, high efficiency, and both small and large area implementation. Nanostructured photovoltaics encompass a wide range of technologies including, but not limited to, intermediate bandgap, quantum dot composites, small molecule and polymer organic, and radial junction. Advantages of quantum dot solar cells include high efficiency through multiple carrier generation (resulting in enhanced photocurrent), decreased cost, and relative ease of fabrication. Both stand-alone and polymer/QD hybrid architectures have been reported. Although current QD cells suffer from low efficiencies, the technology appears quite promising and has prompted further investigation.
Typical colloidal quantum dot compositions consist of an active inorganic core (e.g., CdSe or CdS) shrouded by an organic ligand capping layer (e.g., trioctylphosphine oxide (“TOPO”)). An alternative colloidal QD composition includes an active inorganic core, encased by an inorganic shell (e.g., ZnS or CdS), which is also shrouded by an organic ligand capping layer. Such a composition is referred to as a “core-shell structure”. The inorganic core is solely responsible for emission of light, following either photoexcitation or electrical pumping. In general, core-shell structures possess increased stability and emission quantum yield due to elimination of the core surface defects by the shell moiety. The organic capping layer assists in enhancing the dispersability of the QD composition in various solvents and also acts as a stabilizing agent. As such, it is an integral part of the colloidal system.
However, due to its electrically insulating characteristics, the organic shell can introduce a large barrier to charge injection in electrically pumped solid state systems such as QD-LEDs. Previous efforts have addressed this issue through exchanging the insulating organic ligand (e.g., TOPO) with less insulating organic ligands (e.g., thiols) immediately following the synthesis of colloidal QDs. However, this method has in the past resulted in reduction of emission from the quantum dots due to the exchange procedure and ultimately does not eliminate the organic capping layer and the large barrier to charge injection.
As such, there exists a need for a method and process resulting in elimination of the organic capping layer, post QD deposition.